1. Technical Field
The present disclosure relates to a silicon electrostatic micromotor with indentations, in particular for atomic-level storage systems (generally known as “probe-storage systems”), to which the ensuing treatment will make reference, without this implying any loss of generality.
2. Description of the Related Art
As is known, storage systems that exploit a technology based on magnetism, such as for example hard disks, suffer from important limitations as regards the increase in the data-storage capacity and the read/write speed, and the decrease in their dimensions. In particular, a physical limit exists, the so-called “superparamagnetic limit”, which hinders the reduction in the dimensions of the magnetic-storage domains below a critical threshold, if the risk of losing the stored information is to be avoided.
In the last few years, alternative storage systems have consequently been proposed, amongst which the so-called probe-storage systems have assumed particular importance. These systems enable high data-storage capacities to be achieved in reduced dimensions and with low production costs.
In brief, probe-storage systems envisage the use of an array of transducers (or probes) fixed to a common substrate and each provided with a respective read/write head. The array is positioned in use above a storage medium (e.g., made of polymeric material, ferro-electric material, phase-change material, etc.), and is mobile with respect thereto. Each probe is thus able to interact locally with a portion of the storage medium, for writing, reading or erasing individual information bits. In particular, the relative movement between the storage medium and the array of transducers is generated by a micromotor coupled to the storage medium.
In this connection, electrostatic micromotors are known for generating a linear movement, that are made using technologies of micromachining of semiconductor materials (the so-called MEMS technologies). These electrostatic micromotors base their operation on a capacitive interaction between a fixed substrate (known as “stator”) and a mobile substrate that is able to move with respect to the fixed substrate (known as “rotor”, without this term, however, implying the presence of a rotary movement).
The rotor substrate is generally suspended over the stator substrate via elastic elements. Electrostatic-interaction elements carried by the rotor substrate and stator substrate, for example, rotor and stator electrodes arranged on respective facing surfaces, determine, when appropriately biased, a translation relative movement of the rotor substrate with respect to the stator substrate in a sliding direction.
In particular, the stator electrodes and rotor electrodes form capacitors with plane parallel faces that are misaligned. When a suitable biasing voltage is applied between these misaligned faces, an electrostatic interaction force is generated, which tends to bring them back into a position of alignment, determining the resulting movement of the rotor substrate with respect to the stator substrate. In greater detail, the capacitance of the capacitors formed by the stator electrodes and rotor electrodes varies according to their relative position and in particular is maximum when the electrodes are aligned and minimum in the case of complete misalignment.
A possible trend of this capacitance is shown in FIG. 1a. This trend is periodic, and has a succession of stretches increasing in a substantially linear way from a minimum capacitance Cmin to a maximum capacitance Cmax, the value of which depends substantially on the geometry of the electrodes and their distance apart, and stretches decreasing, also in a substantially linear way and having the same slope as the increasing stretches, from the maximum capacitance Cmax to the minimum capacitance Cmin.
In a known way, the energy stored in the capacitor is given by the expression:
  E  =            1      2        ⁢                  C        ⁡                  (          x          )                    ·              V        2            where C(x) is the capacitance that varies in the sliding direction x, and V is the biasing voltage applied between the stator electrodes and rotor electrodes, having, for example, a pulse-train pattern with rectangular pulses (FIG. 1b).
The component of the electrostatic interaction force along the sliding direction x is given by the expression:
      F    x    =                    ⅆ        E                    ⅆ        x              =                  ⅆ                  (                                    1              2                        ⁢                                          C                ⁡                                  (                  x                  )                                            ·                              V                2                                              )                            ⅆ        x            and thus it is a function of the variation of the capacitance C, and in particular of the slope of its increasing/decreasing trend.
According to the direction of the linear displacement that it is desired to generate in the sliding direction x, the biasing pulses are applied alternatively in the increasing stretch (as is shown in FIG. 1b for a single stator electrode) or in the decreasing stretch of the pattern of the capacitance C. It is thus possible to control, via the biasing voltage V, the direction and speed of movement of the electrostatic micromotor.
In addition to the useful component oriented in the sliding direction, which determines the relative movement between the rotor substrate and the stator substrate, the electrostatic interaction force thus generated has a disturbance component oriented in a direction orthogonal to the sliding direction, which tends, for example, to approach the rotor substrate and stator substrate.
This movement is appropriately countered by the elastic elements coupled to the rotor substrate, which for this purpose have a stiffness sufficient for preventing the movement of the rotor substrate in the orthogonal direction.
In a known way, one of the main targets in the development of electrostatic micromotors is to maximize the useful component and minimize the disturbance component of the electrostatic interaction force or, equivalently, to maximize the ratio between useful component and disturbance component.
Furthermore, it is beneficial to guarantee a minimum target of stability of the rotor substrate as regards deformations in the orthogonal direction (e.g., the deformations should stay within a range of +/−200 nm, considering a thickness for the rotor substrate of approximately 400 μm).